The present disclosure relates generally to control circuits, and more particularly to closed loop current control circuits.
Light emitting diodes (LEDs) are current driven devices, meaning that the amount of current passing through an LED controls its brightness. In applications such as automotive lamps, high intensity LED""s can be used in place of more conventional light sources such as light bulbs. However, the LED""s and their control circuits must be closely matched to avoid brightness variation between adjacent lights. This same problem arises in other applications that employ high intensity LED""s or LED arrays, for example traffic signals and the like.
Manufacturers have implemented several solutions to address the need to closely control the amount of current flowing through the LEDs. One solution is to keep a constant current flowing through the LEDs by using a linear constant current circuit. One problem with using a linear constant current circuit, however, is that the control circuit dissipates a large amount of power, and consequently requires large power devices and heat sinks.
A more power efficient solution has been tried which uses a buck-boost regulator to generate a regulated common voltage supply for the high side of the. LED arrays. Low side ballast resistors are then used to set the LED current, and separate resistors are used to monitor the current. This voltage controlled system also requires compensation diodes to allow for temperature changes in LED characteristics, and requires selecting the current ballast resistors to match the current versus brightness characteristics. Unfortunately the current versus brightness characteristics can change with each batch of LEDs and thus component matching becomes a major consideration. Although this method is more power efficient than using a linear constant current circuit, it still requires large power dissipating external components because current is controlled indirectly through voltage. Another known method for controlling the current through an LED array can be found in U.S. Pat. No. 6,198,405, which uses simple inductor buck-boost type circuits to provide open loop current control with peak currents being much higher than the average LED current since current flow through the LEDs is not continuous. This method, however, does not allow operation of high intensity LEDs at full brightness because of the current swings required.
What is needed therefore is a way to control the current through high intensity LEDs which is both efficient and allows operation of the high intensity LEDs at full brightness.